High pressure electric discharge device having electrodes with thorium on the exposed surface thereof



A ril 11. 1967 F. KOURY ETAL 3,313,974

HIGH PRESSURE ELECTRIC DISCHARGE DEVICE HAVING ELECTRODES WITH THORIUM ON THE EXPOSED- SURFACE THEREOF Filed May 2, 1963 JOHN F. WAYMOUTH FREDERIC KOURY INVENTORS BYLQM ATTORNEY United States Patent 3,313,974 HIGH PRESSURE ELECTRIC DISCHARGE DEVKCE HAVING ELECTRODES WITH THORIUM ON THE EXPGSED SURFACE THEREOF Frederic Koury, Lexington, and John F. Waymouth,

Marblehead, Mass, assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed May 2, 1963, Ser. No. 277,634 5 Claims. (Cl. 313218) This invention relates to the fabrication of high pres sure discharge devices and particularly to the fabrica tion of iodine-containing high pressure discharge devices.

Techniques have previously been disclosed for the fabrication of the conventional high pressure discharges devices. Such conventional devices diifer from the iodinecontaining devices in that the former emit radiation of only a few distinct wavelengths whereas the latter emit a large number of very closely spaced lines. The emission color of conventional high pressure discharge devices, generally calledmercury lamps in the art, is a bluish green, but with the iodine-containing devices, a white light is emitted due to the large number of closelyspaced emission lines. Hence the emission color of the iodine-containing lamps is materially improved and they are quite desirable.

But certain problems have presented themselves to the art and we have discovered that the iodine-containing devices suffer quite markedly from the inclusion of any appreciable concentrations of hydrogen in the arc tube. Formation of a stable arc therein is made extremely difi'icult and the starting voltages are raised materially. It is believed that these difficulties are due to the formation of hydrogen iodide which has a much higher vapor pressure than any other iodide present in the system. For every atom of hydrogen present, it is reasonable to expect that there will be an extra atom of iodine in the vapor state. And when the atoms of iodine are present in the vapor state, starting is made more difiicult.

Now it is known to the art of course, that hydrogen should be excluded from the conventional mercury lamps and more practically, the amount should be reduced to the lowest possible limits. But with these conventional lamps, minor quantities of hydrogen such as might enter the arc tube during sealing are not particularly deleterious. However I have indicated, the problem of hydrogen is presented more acutely in the iodine-containing devices.

We have discovered that one of the major sources of hydrogen contamination is the combustion gases produced during heating and sealing of the arc tube. Products of combustion from ordinary burners using either natural or manufactured gas include in addition to carbon dioxide and water, a quantity of carbon monoxide, low boiling hydrocarbons such as methane and ethane, and also the deleterious hydrogen together with some oxygen. I have discovered that thorium metal, which is usually disposed within the cathodes, is an extremely fine getter for hydrogen and hence will absorb this gas it given the opportunity. The thorium cannot be excluded however and in our copending application, Ser. No. 230,- 944 filed on October 16, 1962, we have disclosed that certain specific quantities of thorium are placed in iodinecontaining high pressure discharge devices in order to obtain a white light emission. Now of course, the inclusion of a thorium sliver in the cathodes of conventional mercury lamps is quite well known and widely accepted in the art. For example, in the patent to Gustin et al., No. 2,241,362, an electron emissive cathode containing thorium is disclosed. Patentee teaches that when the so-called thoriated cathodes (a cathode containing a core of thorium) are heated as a result of a glow disbardment and will become incandescent. Upon incandescence of the electrodes, the core of thorium is an emitter of a copious flow of electrons which initiates an arc discharge between the electrodes. However since the device of the art did not contain iodine, the problem of thoriums gettering hydrogen during sealing was not too troublesome. We have discovered however that these types of cathodes suffer from certatin disadvantages when they are used with iodide-containing high pressure discharge devices. As we have explained, the main disadvantage is due to the extremely high hydrogen gettering ability of thorium. In most high pressure discharge devices employing a double wall envelope, the arc tube is formed of a quartz or similar glass having a pair of thoriated refractory metal electrodes, generally fabricated of tungsten, supported on lead-in wire-s and sealed into either end thereof. The scaling is performed by heating the glass to softening with gas burners and then urging softened sides of the glass together upon the electrode support such as disclosed in the prior art or in the copending application of Loughridge, Ser. No. 256,049, filed February 4, 1963 and assigned to the same assignee as the instant application. We have now discovered that when the glass of the arc tube is softened, it is extremely permeable to hydrogen and the gas passes through quite readily. Since the cathode is quite near the point where the gas flame strikes the softened glass, the thorium core contained in the electrodes is also quite near. As the hydrogen containing gas passes through the wall and over the thorium, it is quickly gettered by the thorium and hence included in the envelope. After sealing the arc tube and striking an arc, the hydrogen is expelled from the thorium sliver due to the heat.

In order to solve these problems, we have discovered that the thorium sliver should not be disposed within the cathode during sealing. Rather we have discovered that a non-thoriated cathode, that is a cathode containing no thorium sliver, should be sealed within the arc tube, and after the sealing is completed, the requisite thorium should be dropped into the arc tube through an appended exhaust tube together with other ingredients'which are necessary such as mercury, iodine and/or mercuric iodide. After inclusion of such material and filling the envelope with an inert gas of suitable 'pressure, an arc may be struck in the tube. The are will act upon the thorium metal and cause it to plate out upon the outside of the cathodes through the-cooperative action of the iodine.

Accordingly, the primary object of our invention is to reduce the amount of hydrogen which is trapped within an arc tube.

And another object of our invention is to reduce starting voltages of white light-emitting high pressure electric discharge devices containing iodine.

Another object of our invention is to eliminate hydrogen gettering by thorium during arc tube sealing.

A feature of our invention is the inclusion of a quantity of thorium metal within the arc tube after the electrodes have been sealed therein.

And yet another feature of our invention is dropping a quantity of thorium metal into the arc tube while the arc tube is under a vacuum through an appended exhaust tube disposed on the side of the envelope.

Other objects, features and advantages of the instant invention will become manifest to those versed in the art upon reading the following specification wherein preferred embodiments of the instant invention are described by way of illustrative examples. The figure is a crosssectional view of'a' high pressure discharge device which can be fabricated according to our invention.

Briefly, the practice of our invention involves the fabrication of non-thoriated electrodes for use in the arc tube.

, will emanate.

Non-thoriated electrodes are quite similar to those generally used by the art since they are fabricated of a shank of tungsten surrounded at one end with a tungsten wire helix, however they differ because they contain no sliver of thorium. At the other end of the shank, a small sheet of molybdenum is attached which in turn is Welded to a lead-in wire. This non-thoriated electrode is then positioned Within the end portion of a tubular envelope usually prepared of a silica glass, such as quartz or Vycor. In order to seal the electrodes into the envelope we then heat the glass to softening at the end in which the electrode is disposed and when soft, a press-seal is made. Conventionally, a stream of a suitable inert gas is directed into the envelope through an exhaust tube during heating so that an inert blanket is provided over all metal parts to prevent their oxidation. When the seal is made, the electrode is rigidly disposed in the envelope.

While continuing the flow of inert gas, the hot sealed end is cooled and a similar non-thoriated electrode is disposed at the other end of the envelope and then sealed in a manner such as described above. After the second sealing, the arc tube is ready to be cleaned and then filled with are forming ingredients and gases.

Since even the non-thoriated electrodes frequently tend to absorb gases which are detrimental to the operation of the are, particularly hydrogen, and furthermore are sometimes coated with carbonaceous particles, we have found that purification is necessary. To effectuate this purification, we have found it highly desirable to add a gas in which an arc can form and then strike the arc. Its heat will cause the hydrogen and other occluded gases together with residual carbon to be driven into the arc tube from where easy removal is possible. Usually such removal can be attained by placing the arc tube under vacuum and then filling the envelope again with a purge gas. The operation can be repeated again if desired and the arc tube is then in condition for the addition of thori um, mercury and iodine, it being possible to add the latter two materials either as elements, the corresponding compound or as a mixture of the elements and the corresponding compound. Of course, other metals or chemicals may be added similarly if desired.

While the arc tube is under vacuum the chemicals mentioned above are added to the arc tube through the exhaust tube. A conventional fill gas can then be added although it is possible to add the gas before the chemicals are added and in this latter case of course, they will not be added while the are tube is under vacuum.

The arc tube may then be fully fabricated merely by sealing off the exhaust tube as is conventional in the art. At this point however, the thorium must be transferred to the cathodes since its disposition there is required.

.Upon striking an arc between the electrodes when the thorium is in this position, a copious flow of electrons Such transfer is attained by forceably striking an arc and having the thorium metal react with the iodine atoms. The thorium iodide will then migrate back to the electrode where it will dissociate and the thorium will plate out and gradually form a ball tip. Operation of the lamp for 5 to 30 minutes will cause substantially all of the thorium to be transferred from the arc tube to the cathode.

Particularly describing our invention, the arc tube is fabricated according to techniques set forth in the abovementioned Loughridge application. We then pump down the arc tube to a partial vacuum and fill with argon, which operation is generally repeated two times. The :argon may be added to a pressure of 300 mm. of Hg pres- ;sure. The purpose for these flush and fill steps is to remove any water and other gases which accidently may be included in the arc tube.

When the arc tube has been flushed and filled with :argon it is then placed under a vacuum and baked at temperatures of 750 to 1200 C. for about minutes until there is no evidence of outgassing in either electrodes or the glass envelope. A convenient measure of outgassing is watching a pressure gauge which indicates the pressure in the arc tube. If outgassing is occurring, the vacuum pump will not be able to maintain a constant vacuum and this inability will be indicated upon the associated pressure gauge. When the gauge is constant, outgassing has ceased. During the baking, the arc tube can be flushed with argon a few times, four for example, to insure the removal of the outgassed gases.

Because additional gases might remain in the electrodes themselves we then fill the arc tube with a small quantity of argon, for example, mm. of mercury pressure, and strike our are between the electrodes. When the arc has :been on for about 30 seconds, it is extinguished and the gases removed. A similar procedure is repeated four more times, the last three times using a larger quantity of argon, for example 200 mm. of mercury. Now it is apparent that many other gases may be used, so long as an arc will form in the tube. Since the arc does not form too readily, starting can be facilitated by applying externally a high frequency, high voltage emissive discharge, which may be for example a Tesla coil. Such coils are prepared by the Ecco High Frequency Corp. and the model G-4 has been found advantageous for our purposes. A current in the order of microamperes is developed together with a frequency in the order of a megacycle while producing up to five to ten thousand volts. It is generally desirable to keep the are formed for a minimum time at high electrode operating temperatures (2000 to 250-0 C.) to prevent inordinate volitization of the tungsten onto the walls of the envelope.

After the arc tube has been cleaned and prepared as described above it is ready to have the chemicals added which will form the white-emitting are. In particular we can add 5.2 10 to 5.2 10- tgram atoms of thorium per cm. arc length, that is 5 to 14 mg. of thorium for a 7.3 cm. are tube (that is the distance between opposing tips of electrodes) having a volume of 25 cubic centimeters although we prefer to add 10 mg. for optimum results. In addition we add all of the iodine which should be included in the envelope, that is approximately 5.2 1() to 1.6 10 gram atoms per centimeter of arc length in accordance with our copending application entitled Electric Discharge Device, Ser. No. 230,944 filed October 16, 1962 and assigned to the same assignee as the instant application.

Usually the iodine is added in the form of mercury iodide with additional mercury metal being-added later to bring the quantity to a point which is adequate for high pressure discharge device arc formation. The two stage addition is preferable since the thorium and particularly mercury iodide tend to absorb small quantities of water which are deleterious to are formation. Such water can be removed easily by flaming the materials while they are disposed in the arc tube and under vacuum and thus causing the mixture to be distilled off and drawn from the envelope. When the flaming is finished, a vacuum can again be formed at which time the remaining quantity of mercury can be added. This amount should be sufficient to form a ratio of iodine to mercury of 0.10 to 0.85 atom iodine per atom of mercury. The final steps in the process are to flush the envelope with argon or a similar gas which is inert to the ingredients of the lamp, then filled with argon at a pressure of about 23 millimeters of mercury and immediately sealed from the atmosphere by tipping off the exhaust tubulation.

Since at least a part of the thorium must be deposited upon the electrodes of the arc tube in order to perform its function, we then start the are at which time the thorium reacts with the iodine and forms ThL, The ThI, then evaporates into the arc and some will dissociate in the vicinity of the electrodes. When such dissociation occurs the thorium metal will plate out upon the electrodes and will eventually form an ever-renewing coating. After operation of the are for about a half hour,

- sufiicient thorium is plated out upon the electrodes to provide the copious flow of electrons which will be needed for subsequent starting by conventional techniques.

The are tube, fully fabricated, is shown in the figure wherein a pair of non-thoriated electrodes are sealed through the ends 12 and 14 of the tube 16. These electrodes may be of any of the typical designs, however we prefer to use ones having a shank of tungsten metal surrounded at their inner ends by a helical winding of tungsten. As illustrated, tungsten shanks 1 and 4 having windings 2 and 3, are sealed into the ends 12 and 14 of the arc tube 16 and attached to one end of molybdenum foil sections 5 and 9. Disposed on the other ends of the foil sections 5 and 9 and extending outside of the arc tube are lead-in wires 6 and 11 which can be connected to a source of current. A residual fused tip 15 which remains after the inclusion of the arc-forming ingredients is disposed upon the outside of the arc tube. After operation of the lamp a thin coating of thorium metal (not shown) is formed upon the electrodes and generally a small ball 17 of thorium will occur at the distal ends. The ball is formed by the dissociation of thorium iodide (ThI in the high temperature arc stream. When an arc tube containing thorium and iodine is operated, the thorium reacts with the iodine to form thorium iodide. Eventually as the temperature and pressure increases, the thorium iodide dissociates and deposits thorium metal upon the hottest place on the electrode.

When fabricating the electrode, it should be designed so that the thorium reacts with the iodine and hence is removed from the wall and deposited upon the electrode. Hence the point of minimum vapor pressure of the thorium or its iodine compounds should be on the electrode. In order to attain such minimum vapor pressure it is necessary to take both the vapor pressure and the stability of thorium iodide into consideration. When plotting the partial pressure of thorium or thorium compounds over metallic thorium in an atmosphere of iodine vapor as a function of temperature, a three component curve results. At low temperature, that is up to a point where the rate limiting reaction process is the arrival of iodine atoms at the surface of the thorium metal, the partial pressure of the Th1; atoms increases with increasing temperature. At higher temperatures, the rate-limiting process is the rate of arrival of iodine atoms at the surface, and the pressure of ThL; becomes a constant, independent of temperaturre. At higher temperatures still, the Thl is unstable and the effective partial pressure of the ThI will decrease. And at higher temperatures yet, evaporation of the thorium metal atoms will take place and the vapor pressure of thorium will increase with increasing temperature.

The range of operating electrode temperature for our lamp is in the first, second or third temperature ranges noted above and hence there is a tendency for the thorium to deposit at the point of highest temperature on the electrode, provided that the partial pressure of thorium corresponding to the operating temperature is less than the partial pressure of the ThI at the arc tube wall. When such conditions are fulfilled, the iodine will pick thorium in the arc tube and gradually deposit it during operation as a ball on the tip of the electrode. Hence it is the drop of thorium metal which serves as the electrode and because of its presence,-lamp efficiency and red rendition stay at high values because of the maintenance of the critical amount of thorium in the arc stream.

It is apparent that modifications and changes may be made within the spirit and scope of the invention without departing therefrom. For example, it is quite possible to add materials with the thorium and mercury iodide which will modify the emission color of the arc or broaden the discharge such as adding cadmium for increased red, thallium for increased green or sodium to change the shape of the arc. Hence it is our intention to be limited only by the following claims.

As our invention we claim:

1. A high pressure electric discharge device comprising an arc tube; a filling in said are tube comprising mercury atoms and iodine atoms; a pair of electrodes disposed at either end of said are tube, said electrodes having a coating of thorium metal disposed upon the exposed surface thereof in order to supply thorium atoms to the are discharge.

2. A high pressure electric discharge device comprising: an arc tube, an electrode disposed at either end thereof, each of said electrodes having a central shank portion surrounded'by helical Winding at the inner ends, a filling in said are tube of mercury and a iodine and coating of thorium metal disposed upon the exposed surfaces of said electrodes in order to supply thorium atoms to the arc discharge.

3. A high pressure electric discharge device comprising: an arc tube; an electrode disposed at either end thereof, each of said electrodes having a coating of thorium metal disposed upon the exposed surfaces thereof and a ball of thorium disposed on upon at least one of the distal ends in order to supply thorium atoms to the arc discharge, a filling in said are tube of mercury and iodine.

4. A high pressure electric discharge device comprising: an arc tube; an electrode disposed at either end thereof, each of said electrodes having a coating of thorium metal disposed at the exposed point of maximum temperature in order to supply thorium atoms to the arc discharge; a filling in said arc tube of iodine and mercury.

5. The device according to claim 4 wherein part of the thorium coating is in the shape of a ball disposed at the distal end of at least one of the electrodes.

References Cited by the Examiner UNITED STATES PATENTS 2,691,853 10/1954 Yoder 2925.13 2,740,186 4/ 1956 Gates 2925.13 2,916,653 12/ 1959 Macksoud 313346 3,153,169 10/1964 Bauer 313161 3,170,081 2/ 1965 Rokosz 313-213 3,234,421 2/ 1966 Reiling 313-25 JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, DAVID J. GALVIN, R. JUDD,

D. E. SRAGOW, Assistant Examiners. 

1. A HIGH PRESSURE ELECTRIC DISCHARGE DEVICE COMPRISING AN ARC TUBE; A FILLING IN SAID ARC TUBE COMPRISING MERCURY ATOMS AND IODINE ATOMS; A PAIR OF ELECTRODES HAVING A COATING OF THORIUM METAL DISPOSED UPON THE EXPOSED SURFACE THEREOF IN ORDER TO SUPPLY THORIUM ATOMS TO THE ARC DISCHARGE. 